1. Field of the Invention
The present invention relates to a pneumatically static balancer for a machine tool to be used with a driven member of a vertical shaft.
2. Description of the Background Art
In a vertically moving drive device such as a main spindle head in a vertical machine tool or a saddle of a horizontal machine tool, there is known a balancer which has such a balancer mechanism as is compatible to weight of a driven member so as to prevent the driven member from falling or to apply no load to a motor at a stopping time. This balancer is categorized as a counter balancer type, a liquid pressure cylinder type using oil or water pressure, an air pressure cylinder type or the like. In these kinds of balancers, for aiming at reducing the space, a hydraulic cylinder, which is arranged coaxially with the driven member, is known.
FIG. 7 is a conceptional diagram showing a known balance mechanism of a vertical machining center as a machine tool. As shown in FIG. 7, a machine tool 101 is provided with a driven member 102 which vertically moves against the gravity. By a rotating drive of a drive mechanism 103 of a motor, a feed screw 104 is rotated to move the driven member 102 vertically. In a bed below the driven member 102, there is disposed hydraulic cylinder 111, from which a cylinder rod extends to support the lower face of the driven member 102.
In a balance mechanism 110, the pressure of oil to be fed from a pressure source 112 to the hydraulic cylinder 111 is adjusted by pressure adjusting means 113 to such a pressure P1 that the force to act on the piston of the hydraulic cylinder 111 is adapted to counterbalance a gravitational load F1 of the driven member 102. The excess oil to be fed from the pressure source 112 to the pressure adjusting means 113 is circulated to a tank 115 on the side of the pressure source 112 through a drain 114.
However, frictional resistance of a seal member disposed between the piston of the hydraulic cylinder 111 and the hydraulic cylinder 111 obstructs the action of the driven member 102. In case the driven member 102 is subjected to a slow or extremely minute feed, moreover, a stick slip phenomenon is caused due to the difference between a dynamic friction resistance and a static friction resistance to act between the piston of the hydraulic cylinder 111 and the outer cylinder of the hydraulic cylinder, thereby to raise a problem that the driven member cannot be highly precisely positioned. Since the feeding action becomes unstable, moreover, it has been desired that the resistance to the drive of the balancer be minimized.
FIG. 8 is a schematic diagram showing a balance mechanism of another known vertical machining center. In the balance mechanism having improved the aforementioned problems, as shown in FIG. 8, a machine 122 is constituted such that a driven member 125 is vertically fed by a feed screw 124 rotationally driven by a motor 123. The driven member 125 is exemplified by a saddle, a working head or the like in a machine tool and is vertically moved along a post 126, as called a column or the like.
On the upper and lower portions of the right side of the post 126, there are fixed two sliding support members 127 and 128 which constitute a pneumatically static guide mechanism. The driven member 125 is attached so vertically slidably as is fitted in those sliding support members 127 and 128. In the inner circumferences of the through holes of the sliding support members 127 and 128 passing the driven member 125 therethrough, there are formed circumferential grooves 129 and 130 which are individually divided into three or more circumferential portions. In interiors of the sliding support members 127 and 128, there are formed conduits 131 and 132 of the same number as that of the circumferential grooves 129 and 130. These conduits 131 and 132 communicate with the circumferential grooves 129 and 130, respectively. The conduits 131 and 132 are connected to a pressure source 135 composed of an air compressor and so on through external conduits 133 and 134, respectively.
Moreover, a closed chamber 138 of a bottomed container shape is so integrally fixed to the sliding support member 128 as to enclose the lower end portion of the driven member 125.
In order the driven member 125 may not receive the frictional resistance of the seal member, the closed chamber 138 reserves the air having leaked from a static pressure bearing portion, in the chamber enclosed by one driven member by using the slide faces of the sliding support members 127 and 128 as the static pressure bearing. The closed chamber 138 supports the driven member as a kind of air pressure cylinder.
However, because the above-described apparatus support the gravitational load by the air pressure fed to the one outlet closed chamber of the pneumatically static guide mechanism, a larger chamber than the driven member is required thereby to raise a problem that the device is enlarged (as referred to Japanese Patent Unexamined Publication No. JP-A-10-138081, Paragraph Nos. 0007-0020, FIG. 1 and FIG. 6).